6.8 SPC vs. 300 BLK – Which Way to Go in an AR Platform Rifle?

Longtime readers know I’m a big fan of the 300 BLK platform. With the increased popularity of 6.8 SPC ammunition, and given the Army’s new fondness for the round, I’m asked if 6.8 SPC is really the way to go in an AR platform rifle. That’s a valid question — what makes the 300 AAC Blackout round so much better than the other calibers available for the AR platform? And is it actually better? Like so many questions when it comes to guns…

It all depends on what you want to do with it.

With the apparent adoption of the 6.8 bullet by the military, the round is bound to increase in popularity and availability. So to look at 6.8 SPC vs. 300 BLK — two great rounds — I’m going to break this into three categories, encompassing everything a shooter should consider when choosing a caliber: performance, compatibility and availability.

PERFORMANCE

The 6.8 SPC and 300 BLK cartridges were designed for two very different jobs. 6.8 Remington SPC was designed to be a longer range cartridge, while 300 BLK (AKA 7.62×35) was designed to be a shorter range cartridge that works well suppressed. I interviewed Kevin Brittingham, founder of AAC (and godfather of the 300 BLK cartridge) and he had this to say about his round:

These were our original requirements for this caliber: Muzzle energy has to equal or exceed the AK-47. .30 caliber projectile. Use unmodified 30 round magazines to full capacity. Use unmodified AR-15/M-16/M-4 bolt. Gas impingement system. Shoot super and subsonic. And one thing that was nice, but was not a ‘deal killer’, was non-adjustable gas system. Cycle all four ways – subsonic suppressed and unsuppressed, and supersonic suppressed and unsuppressed. …

It’s not one of the 6.5 or 6.8 or whatever is the flavor of the month where it’s a do-all, kick-ass… From a military standpoint, here is the advantage (which we discussed earlier): all we need is to change the barrel and get the ammo.

Non-adjustable gas system… And that’s a huge thing. Nobody works with silencers more than us. Every time I’m working with a company on something, whether it’s LWRC or Colt or FN, whatever it is, if it’s got an adjustable gas system we get halfway through the test and we’re like “holy shit, we didn’t adjust the gas system.”

It’s not something that’s really intuitive, you gotta think about having a suppressed system or unsuppressed. If you can get away with a non-adjustable it’s always best. And we’re able to achieve that, and it’s because we did the whole gun; we did the ammo and the silencer so it all works together.

So, 300 BLK was designed to fill a specific role. It was designed to be a big, heavy round that can be quickly changed from subsonic to supersonic and suppressed to unsuppressed.

6.8 SPC, on the other hand, was never intended to be particularly quiet. The relatively small projectile was designed to add a little more mass to the round while maintaining that zippy velocity that gives 5.56 ammo its punch. And thanks to that extra velocity, the 6.8 SPC does indeed produce better ballistics at longer distances.

Out at 500 yards, 300 BLK drops 30 more inches compared to 6.8. But to 200 yards, the difference is less than two inches.

That higher velocity also means that the 6.8 cartridge has a higher muzzle energy than the 300 BLK, clocking in at about 1,694 foot-pounds for the 115gr variant to 300 BLK’s 1,360 for their 125gr offering. And the 6.8 SPC’s performance only improves with added bullet weight. (By the way, 5.56 NATO 55gr comes in at 1,188 foot pounds.)

So, in short, if you really want the most muzzle energy and long distance performance, then 6.8 is the way to go. Actually .338 Lapua Magnum is the way to go, but that won’t fit into an AR-15 magazine.

COMPATIBILITY

To me, the entire point of having a funky caliber in the AR-15 platform is to be able to switch quickly and seamlessly from one caliber to the other. I should be able to swap uppers and be good to go. And, in general, that’s possible with both calibers. But one caliber really does it better.

The 6.8 SPC cartridge is designed from the .30 Remington cartridge, meaning that it won’t work with a standard AR-15 bolt. You need a slightly larger bolt, which means less material on the bolt face to contain the force of the escaping gasses and a higher probability of failure (less material + higher muzzle energy…). And when it fails, you’ll need one of your special snowflake bolts instead of any old standard AR-15 bolt.

Speaking of special snowflake parts, while the 6.8 SPC round will fit in an AR-15 magazine, it’s actually too fat to work at full capacity. Most manufacturers appear to warn users to only load five or six rounds to avoid bulging the magazine. Magpul makes a 6.8 magazine, but it only works with LWRC Six8 rifles.

The headache doesn’t stop there, though. There are, by my count, four incarnations of the 6.8 caliber. One is the SAAMI spec 6.8 Remington SPC, which is a lower pressure load. Then there’s SPC II and 6.8×43, both of which have different chamber pressure ratings. And finally there’s Noveske’s Mod 1 design, which apparently allows the highest pressures and best performance — but its proprietary, and not cheap.

In short, you need to double-check that you’re buying the caliber you want before you drop the coin on that new upper, and ensure that you’re getting the right ammo.

300 Blackout is a simpler proposition. There is only one flavor, which is an open and freely available SAAMI spec that’s already rated to a higher chamber pressure than its wildcat predecessor (the 300 Whisper). Which means that if you get a 300 BLK barrel from any manufacturer you can rest assured that its going to take any 300 BLK ammo you throw at it and more.

Nick Leghorn for TTAG

300 BLK also removes a lot of the equipment headaches. Since it is made with 5.56 brass and was designed specifically to work in standard AR-15 magazines, it works flawlessly with standard AR-15 bolts and magazines. There are no issues with feeding, no issues with magazine capacity, and spare parts are abundant since the only real difference is the barrel.

So, if you’re looking for a simple drop-in option for your AR-15 that works with all of your existing gear and spare parts, then 300 BLK is the way to go.

AVAILABILITY

Having a gun chambered in a kickass caliber doesn’t matter if you can’t find any ammo for that gun. And, right now, 6.8 SPC was far ahead, but 300 BLK has come a long way.

6.8 SPC is just about everywhere — the only place I still can’t seem to find it is Wal Mart. Its been about a decade since the first 6.8 rounds started appearing, and so just about every manufacturer has a load available and ammo in stock. And, thanks to the community of shooters using 6.8 SPC already and the Army’s decision to adopt the round, there’s no doubt that this round will continue to be widely available in the future as well.

300 BLK is the new(er) kid on the block. It hasn’t been adopted en masse by military or law enforcement, so there are no milsurp loads available. But while the “professionals” might not have adopted it, I’ve seen more and more hunters adopting the round as a way to quickly change from their 5.56 M4gery to their hunting rifle.

Down here in Texas where hog hunting is huge, the ability to switch between subsonic and supersonic and suppress the ammunition effectively is a big plus in a hunting round. That use case has led to a lot demand for guns chambered in caliber, and now just about every manufacturer makes a gun in 300 BLK.

In the end, though, it’s a tie for availability. 6.8 SPC is already widely available and that will probably only improve. With the steady growth in popularity of 300 BLK, there’s no shortage of great options in 300 BLK ammunition, either.

SUMMARY

As I said at the beginning, it all depends on what you want to do.

If you’re looking at muzzle energy and long range performance, then 6.8 SPC is the clear winner. It’s a field tested and battle proven cartridge with a huge following that can put animals down at longer distances than 5.56 NATO. And if you’re comfortable with the slightly more complex world of parts and ammo, then go for it.

But if you’re looking for something for hitting critters up to 200 yards away, does it better than 5.56, uses all your existing gear, can swap easily from supersonic to subsonic and is easily suppressed, then 300 BLK is the winner. It may not be a great long distance round, but the compatibility with existing gear really was the feature that sealed the deal for me.

I like 300 BLK. But then you probably knew that already. If I wanted something harder hitting or for longer distances, 7.62 NATO would be my choice.

I wouldn’t turn your nose up to 6.8 SPC, the issue with it is it’s nowhere near as popular or cheap as x39 is. Even with brass case PPU, the x39 is cheaper than 6.8 is, in some cases nearly 33% less. Factor in the steel case x39, the price difference is astonishing and the accuracy I’ve seen some of that 7.62×39 steel ammo gets from the AR is amazing.

Unless the 6.8 is half MOA with even the cheapest ammo, I can’t see how its better than x39 unless variety of hunting ammunition or bullets and sectional density is the utmost importance to someone.

Remember, the 6.8 exists for one reason: extra power and range in their M-16’s and M4’s. The military can’t run 7.62×39 in their rifles because the guns and mags aren’t compatible and won’t pass the tests. The KS47 is purpose built to run x39, from AK mags, and use a few of the more common AR parts that people want to customize, namely the trigger, stock, grip. For the civilian shooter, it’s the best .30 caliber intermediate rifle that can be bought that shoots the most affordable ammo.

But, there’s people out there who have to brag about how much more their 6.8 rifle or .300 rifle cost them and the ammo cuz they don’t put a price on their life.

5.56 ain’t goin anywhere period. The US military, wether anyone here likes it or not, will be using 5.56 in an AR platform until firearms are no longer the primary infantry weapon. I give it another 100 years minimum.

This. 5.56 has the ballistics to real infantry ranges, it’s easy to use and it’s established most of all. It’s got enough umph to render combat ineffective in most situations and when it doesn’t it’s light enough to have more to sling that way. That’s all they care about. If you want the armies take on real world ballistic performance see their sniper weapons. Those are niche systems though.

The Army is going to adapt a new 6.8 round for front line troops. Its not the 6.8 SPC, it is a totally new redesigned round. Why, to defeat body armor, which the 5.56 can not do unless it is tungsten tipped.

Yeah, anyone who thinks the military is going to change calibers isn’t familiar with the reality of how it treats weapons procurement. They loooove testing and the idea of ‘improvement’ but avoid actual changes like the plague. Look at how long it took them to change from the M9, and that’s just a sidearm (and they maintained the same caliber) that is nowhere near as important as the primary weapon.

The 6.8 SPC is a crappy solution to a problem that is completely fabricated.
Google bolt thrust, chamber pressures of Mk 262 vs 6.8, hell, just do the freaking math about replacing EVERYTHING in inventory to switch to a cartridge that will just cost more and you can’t carry as much.
The Green hat wearing guys can do whatever they’d like, but someone needs to stop the OER bulletpoint hunting Field Grade morons at Building 4!

It really has changed…
I got to see the transition in stages. Basic in ‘02 it was at its original configuration, back a couple times through the early 2000s, then as an Instructor with C co 2-29 IN 09-11 the building was 100% scaffolded and stripped. That time frame nearly everyone lives like nomadic hermits, construction everywhere.
I was back two years ago, odd. Very, very odd. One of the strangest parts about Benning is during times of scale up and war it’s crowded, nut-to-butt. Today it’s kind of a ghost town.

I agree, and throw in .30-06 as well. If someone is looking to replace the US Military round, I’ll leave it to them to decide whether or not the intended targets are worth a single round or multiple. For my own purposes, I have a pile of AR platform rifles and carbines to shoot for fun. They were also fun to build up. If I’m planning on hunting or serious shooting beyond handgun range, however, I’ll take the real .30 cal stuff, or larger. Just old fashoined, I guess.

Nice of him to give a shout out to JDJones, the guy who first wildcatted the .300 BLK, calling it the .300 whisper. He made a .30 whisper too. That one never got SAAMI specs and is still a wildcat.
JDJ prototyped some gas-operated autoloading pistols using standard .44 magnum ammo called the “Magmatic” back in circa 1980, too, but it never went anywhere. He’s still mostly known as the guy who makes the giant caliber T/C Contenders.

If time money and space were unlimited I’d probably have one of each and every other gun that has ever tickled my fancy. Since such things aren’t possible, and since I already have an AR in 5.56, I guess my answer would be that I don’t see a compelling reason for either.

If you make a mistake loading your magazines, packing your range bag, or otherwise have a brain fade, having both 5.56 and .300 BLK in the same house can lead to a kaboom moment if you don’t realize you cross-chambered a round. (We’ve all seen the photos.)

5.56 and 6.8? Not so much.

By the way, here’s SAAMI’s list of “don’t put this cartridge in that chamber” :

Yeah, other way around makes sense. Some combos I certainly would never have thought of on that list. I’m also a bit surprised that there aren’t more bad combos listed under 30-06 given how many different cartridges are derived from it.

Sounds like the Phillips & Rodgers Medusa Model 47 Multi-Caliber Revolver, based on the Smith & Wesson “K” Frame. Could chamber up to 25-different calibers, starting with the .38 Special. Don’t know whether anyone ever tried chambering the .40S&W into it or not…

6.8 was not meant to be a long range round; it has worse external ballistics than 5.56. It was meant to be a better manstopper. I doubt that this thing about the US switching to 6.8 will go anywhere in the near future, and if they do end up with a 6.8mm cartridge it will be more powerful than 6.8 SPC, with the ability to use 130 grain bullets.

Depends on what you’re measuring. The .308 bullet is 7.82mm, which matches the groove diameter. The land diameter is 7.62mm, so both names are accurate. Similarly, the 6.8SPC has a 7mm bullet with 6.8mm land diameter.

Yes, I know and that is why I mentioned cartridge naming conventions – the dimensions are what they are but the way we refer to them has become convoluted over time. In one case we are referring to the bullet, in another, the barrel, with early cartridges we’re often referring to the case, etc. If you were to ask someone, for instance, which uses a larger bullet, .308 Win or .303 British, most uninitiated people would guess wrong.

There are many manufacturers of 6.8SPC magazines, they are just steel, not polymer.

Also 6.5 Grendel is a better round past about 300yds. 6.5 Grendel also has less fragile bolts.

And steel cased ammo.

And close enough in hard hitting at close ranges.

6.5 Grendel, better round than 6.8SPC.

300BO, is not the equal to 6.5G, 6.8SPC or 7.62×39 for muzzle energy or close range energy. Typically 300BO is around 1300-1400ft-lbs, 6.5G, 6.8SPC and 7.62×39 are all roughly 1500-1650ft-lbs. It is only past about 75-100yds that 300BO delivers more energy than 7.62×39.

And at no range is 300BO more energetic than 6.8/6.5. It does have subsonic going for it. That is about it. I guess for very short range work (under 100yds) it might be a better options, but a 123/130gr pill at about 2500-2600fps at 6.5/6.8mm in diameter is possibly a better option than a 110gr 7.62mm diameter bullet only traveling about 2200fps (and in both cases, the 300BO by 100yds is more like 2000fps and the 6.5 and 6.8 are still ticking along at 2350-2400fps).

The only polymer 6.8 SPC magazine I know of is unfortunately proprietary for LWRC’s Six8 series of rifles, and they had to change the interior dimensions of the lower and upper receivers to accommodate it. It is, however, a proper 30-rounder that fits well in AK pouches and allows COALs of 2.32″. The same would have to be done for any 6.5 Grendel poly mag for an AR-15, and the dimensional changes would be greater than the former. This is why I’ve been aching to see a mass-produced 6.5 Grendel AK — I’d snatch one up in a heartbeat if my budget allowed it.

As far as external ballistics goes, there’s no significant differences between the 6.5 G and 6.8 SPC until you get to 400-500 yards, and beyond that is where the 6.5 G really shines especially in a DMR application. Shifting to internal ballistics, the 6.5 G actually requires an AR-15 bolt to be blown out even further than for the 6.8 SPC, making them just as fragile without a thorough redesign because of it’s wider rim and base diameter.

The 6.8 is far more available in general, and the 6.5 G is only cheaper in its steel-cased variety. Indoor ranges in particular don’t seem to like steel-cased ammunition because it’s typically (but not always) loaded with bi-metal jacketed bullets, which pose a fire hazard for their bullet traps thanks to their propensity to spark.

You’ll get at least 200 FPS more velocity out of the 6.8 than the 6.5 G regardless of bullet weight or barrel length, so it’s always going to hit harder at close range.

6.8 SPC, better round than the 6.5 Grendel unless you want to reach out and touch someone.

What attracts people to the .300 BLK is the fact that you only have to change out a barrel to use it, and is close enough to the 7.62x39MM at short range for it not to matter all that much.

Did anyone ever try just sticking a 6.5mm bullet in a resized 5.56 case? Basically make a .264BLK? Seems you’d have a winner if you could get 95% or better of the performance of a 6.5G round but keep it as simple to convert as it is to go to a 300BLK. Also, whatever happened to the 6x45mm?

The problem is case capacity. The Grendel bullet on average weighs twice what a 556 does (62g v 123g). Not enough powder to fling it. It’s why a grendel mag holds 25 rounds: bigger case with enough powder to provide the KE needed.

For me .300blk makes sense when you consider it as part of a system–suppressed and unsuppressed SBR without needing an adjustable gas system. With my SBR it’s also the softest recoiling of the three mentioned in the article. Once you leave the system in some way it loses its attractiveness and utility.

I doubt .300blk will be accepted in the military due to safety reasons and because they don’t have to worry about the issues we do. For a civvie like me, though, it’s an awesome caliber and I love it!

Oh, just stop. Yes, you can boom 300 out of a 5.56 barrel. And yes, you can just as easily violate one of the four firearm safety rules. By the rational being punted around here, half you shouldn’t own a gun, let alone use 300blk for anything. AHMYGawdcouldfitinside556! Seriously.

You’re adults. Act like one and take responsibility for your own safety.

I would think 6.5 Grendel would be the best cartridge for regular infantry. M4s, SAWs, and even DMRs could be chambered for it. Much better lethality and range than the other candidates. The 300 BO would be ideal for Special Forces in clandestine ops requiring a lower signature. It could also be used for urban ops. I’d still prefer 6.5 G for that role with a suppressor. Especially, if encountering armored enemies.

What has me finally considering a 300 BO is it’s utility in short barrels on braced pistols, both sub and supersonic, at my real world ranges using bullets that are commonly available or easily cast, common powders and cast-off cases… 6.8 SPC doesn’t do most of that for all I can see. 300 BO seems like a reloaders dream cartridge. Am I wrong? Please convince me I don’t need one…

I personally just don’t like the 556 bullet to light for brush type hunting I use a old norinco SKS little heavy that does not matter in tree stand and if I need a gun for longer distance hunting I break Out the old remington mod.760 in 30 06 because it comes in loads from 165 grain up and if I need anything different will hand load like I can do with 7.62×39 because they come in two weights 123 & 154 SP other than that its a 22 or shotgun or the AK and its a long barrel maddi

To me, the 300 BO, isn’t more than a good laugh. It “shines” subsonic?? LOL So does my 45 acp with the same weight bullet. But…. “It’ll shoot 110 gr bullets 2200 fps”… so will my 357 Mag rifle! LMAO!!! My 9 mm AR nearly does all this AND the rounds are interchangeable with my pistols!! ROTFLMAO!!! For the price of a 300 bo suppressor I just got 2000 rounds of 6.8 SSA BTHP and ear muffs.